Method of measuring focus of a lithographic projection apparatus

ABSTRACT

A method of measuring focus of a lithographic projection apparatus includes exposure of a photoresist covered test substrate with a plurality of verification fields. Each of the verification fields includes a plurality of verification markers, and the verification fields are exposed using a predetermined focus offset. After developing, an alignment offset for each of the verification markers is measured and translated into defocus data using a transposed focal curve.

This application is a continuation of U.S. patent application Ser. No.12/273,772 filed Nov. 19, 2008 now U.S. Pat. No. 8,289,516, which inturn claims priority and benefit under 35 U.S.C. §119(e) to U.S.Provisional Patent Application Ser. No. 60/996,506, filed on Nov. 20,2007. The contents of those applications are incorporated herein intheir entirety by reference.

FIELD

The present invention relates to a method of measuring focus of alithographic projection apparatus. It also relates to a method ofcalibrating such a apparatus using the method of measuring focus.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.comprising part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at one time, andso-called scanners, in which each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

In the manufacture of devices using lithographic processes, each maskpattern is typically projected onto the target portion in focus. Inpractice, this means that the target portion of the substrate ispositioned in a plane of best focus of the aerial image projected by theprojection system. As the critical dimension (CD), i.e. the dimension ofa feature or features in which variations will cause undesirablevariation in physical properties of the feature, such as the gate widthof a transistor, in lithography shrinks, consistency of focus, bothacross a substrate and between substrates, becomes increasinglyimportant.

The use of an alignment system to monitor focus has been proposed andinvolves printing focus-sensitive alignment markers at known positionsrelative to normal alignment markers at various different focussettings, i.e. positions of the substrate relative to the projectionsystem. The position of these focus-sensitive markers with respect tothe normal alignment markers is measured and an alignment offset (AO)can be determined which is representative of focus errors.

The quality of focus control in lithographic tools is today verified byusing the Leveling Verification Test (LVT). A potential benefit of thismethod is that the read-out of the wafer can be done by the alignmentsystem present on the lithographic tool itself. Hence no off-lineread-out tool may be needed. The LVT test uses a special reticle withglued glass wedges on top, to locally create non-telecentricillumination on a double telecentric lens. This non-telecentricillumination is used to cause a lateral shift in x, y as function ofdefocus z of the aerial image of an XPA alignment mark situated beneatha glass wedge. By measuring the alignment shift of this defocus markwith respect to XPA reference mark (imaged without wedge on top), thedefocus at the moment of exposing can be determined.

Up to this point, the current LVT test has been working quite well.However, for future lithographic projection tool designs, threepotential drawbacks of the LVT method may become relevant. The tighterfocus control for new systems put higher demands on the signal to noiseratio of the defocus measurement technique. The read-out noise of thealignment system, and the positioning accuracy of the reference marksare important contributors to the measurement noise of LVT, due to afairly low focus-versus alignment shift sensitivity (typicallyd(X,Y)/dZ=0.4). This low focus versus shift sensitivity can not befurther increased due to restriction of the height and angle of wedges).Secondly, the LVT test has a restricted spatial sampling density due tothe fact that each defocus measuring mark requires a relatively largewedge on top. Finally, and more importantly, the current LVT test methodrequires light being transmitted through wedges. The current LVT testmethod can thus not be applied for future maskless or EUV systems.

SUMMARY

It is desirable to provide a new method for doing leveling verificationtests which has higher focus-versus alignment shift sensitivity andhigher spatial sampling density.

According to an aspect of the invention, there is provided a method ofmeasuring focus of a lithographic projection apparatus, the methodcomprising:

-   -   covering a test substrate with photo resist;    -   placing the test substrate on a substrate table of the        lithographic apparatus;    -   exposing a plurality of verification fields on the test        substrate, each of the verification fields comprising a        plurality of verification markers, wherein the verification        fields are exposed using a predetermined focus offset FO;    -   developing the photo resist;    -   measuring an alignment offset for each of the verification        markers;    -   translating the measured alignment offsets for each of the        verification markers into defocus data using a transposed focal        curve.

According to a further aspect, there is provided a method of calibratinga lithographic projection apparatus, comprising the method of measuringfocus described above, wherein the defocus data is used to adjustsettings of the lithographic projection apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus which may be used in conjunctionwith the invention.

FIG. 2 shows an example of a FOCAL reticle used to expose a plurality ofverification fields on a test substrate;

FIG. 3 shows a possible structure of such a FOCAL marker Mk1 whichcomprises horizontal and vertical chopped bars;

FIG. 4 shows an example of a focal curve;

FIG. 5 shows an example on how a Ry tilt introduces Z height positionsof calibration marks of one particular row of a calibration field as afunction of the X-position within the field;

FIG. 6 shows a graph of a measured alignment offset for one single rowof calibration marks of one calibration field;

FIG. 7 shows possible positions of the calibration fields on a testsubstrate;

FIG. 8 shows an example of a calibration curve.

DETAILED DESCRIPTION

FIG. 1 schematically depicts an example of a lithographic projectionapparatus which may be used in conjunction with the invention. Theapparatus comprises:

an illumination system (illuminator) IL configured to condition aradiation beam B (e.g. UV radiation or EUV radiation).

a support structure (e.g. a mask table) MT constructed to support apatterning device (e.g. a mask) MA and connected to a first positionerPM configured to accurately position the patterning device in accordancewith certain parameters;

a substrate table (e.g. a wafer table) WT constructed to hold asubstrate (e.g. a resist-coated wafer) W and connected to a secondpositioner PW configured to accurately position the substrate inaccordance with certain parameters; and

a projection system (e.g. a refractive projection lens system) PSconfigured to project a pattern imparted to the radiation beam B bypatterning device MA onto a target portion C (e.g. comprising one ormore dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure supports, i.e. bears the weight of, the patterningdevice. It holds the patterning device in a manner that depends on theorientation of the patterning device, the design of the lithographicapparatus, and other conditions, such as for example whether or not thepatterning device is held in a vacuum environment. The support structurecan use mechanical, vacuum, electrostatic or other clamping techniquesto hold the patterning device. The support structure may be a frame or atable, for example, which may be fixed or movable as required. Thesupport structure may ensure that the patterning device is at a desiredposition, for example with respect to the projection system. Any use ofthe terms “reticle” or “mask” herein may be considered synonymous withthe more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g. water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the projection system. Immersion techniques arewell known in the art for increasing the numerical aperture ofprojection systems. The term “immersion” as used herein does not meanthat a structure, such as a substrate, must be submerged in liquid, butrather only means that liquid is located between the projection systemand the substrate during exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDcomprising, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may comprise an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL maycomprise various other components, such as an integrator IN and acondenser CO. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross-section.

The radiation beam B is incident on the patterning device (e.g., maskMA), which is held on the support structure (e.g., mask table MT), andis patterned by the patterning device. Having traversed the mask MA, theradiation beam B passes through the projection system PS, which focusesthe beam onto a target portion C of the substrate W. With the aid of thesecond positioner PW and position sensor IF (e.g., an interferometricdevice, linear encoder or capacitive sensor), the substrate table WT canbe moved accurately, e.g., so as to position different target portions Cin the path of the radiation beam B. Similarly, the first positioner PMand another position sensor (which is not explicitly depicted in FIG. 1)can be used to accurately position the mask MA with respect to the pathof the radiation beam B, e.g., after mechanical retrieval from a masklibrary, or during a scan. In general, movement of the mask table MT maybe realized with the aid of a long-stroke module (coarse positioning)and a short-stroke module (fine positioning), which form part of thefirst positioner PM. Similarly, movement of the substrate table WT maybe realized using a long-stroke module and a short-stroke module, whichform part of the second positioner PW. In the case of a stepper (asopposed to a scanner) the mask table MT may be connected to ashort-stroke actuator only, or may be fixed. Mask MA and substrate W maybe aligned using mask alignment marks M1, M2 and substrate alignmentmarks P1, P2. Although the substrate alignment marks as illustratedoccupy dedicated target portions, they may be located in spaces betweentarget portions (these are known as scribe-lane alignment marks).Similarly, in situations in which more than one die is provided on themask MA, the mask alignment marks may be located between the dies.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the mask table MT and the substrate table WT are keptessentially stationary, while an entire pattern imparted to theradiation beam is projected onto a target portion C at one time (i.e. asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

2. In scan mode, the mask table MT and the substrate table WT arescanned synchronously while a pattern imparted to the radiation beam isprojected onto a target portion C (i.e. a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the masktable MT may be determined by the (de-)magnification and image reversalcharacteristics of the projection system PS. In scan mode, the maximumsize of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the mask table MT is kept essentially stationaryholding a programmable patterning device, and the substrate table WT ismoved or scanned while a pattern imparted to the radiation beam isprojected onto a target portion C. In this mode, generally a pulsedradiation source is employed and the programmable patterning device isupdated as required after each movement of the substrate table WT or inbetween successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

According to an embodiment, a method of measuring focus of alithographic projection apparatus is provided which can be explained asa way to perform a focus/leveling test using the well-known FOCALreticle. First a wafer is exposed with full wafer coverage(verification) fields at a predetermined optimal focus offset FO. Thepurpose of the focus offset FO is to be in the most focus sensitive partof a so-called focal curve, as will be explained in more detail below.

FIG. 2 shows an example of such a FOCAL reticle used to expose aplurality of verification fields on a test substrate. The FOCAL reticlecomprises a plurality of markers Mk1 . . . Mk247, which are named FOCALmarkers. FIG. 3 shows a possible structure of such a FOCAL marker Mk1which comprises horizontal and vertical chopped bars. The bars in thestructures may have a range of line widths and pitch sizes. Due to theparticular dimensions of the structure, the line width of the chops willbe focus sensitive according to the well known Bossung principle. Asresult of this, a FOCAL marker not exposed in focus will have analignment offset. This offset can be used to determine a focus erroralso referred to as defocus.

The verification fields are exposed using a predetermined focus offsetFO. The particular focus offset FO is determined by looking at a focalcurve C(dZ) already exposed on the lithographic apparatus. An example ofsuch a focal curve is shown in FIG. 4, see curve 40. The focal curve,reference number 40, is a graph indicating an alignment offset (ao) ofmeasured FOCAL marks as a function of defocus dZ. In FIG. 4 a usefulfocus offset is shown with reference number 41. Around the focus offseta focus range 42 is indicated in which there is a unique relationshipbetween the alignment offset and defocus dZ. The focus offset FO isdetermined such that all the exposed FOCAL marks will stay within theusable focus range 42 throughout the exposure, see FIG. 4. This meansfor example that the predetermined focus offset FO cannot be too closeto dZ=0 axis since the alignment offset as function of dZ is zero whichis not usable for the method. In an embodiment, the focus offset FO isselected so as to be in the middle of a working range 42 where the focalcurve C(dZ) is substantially linear since this is the region of maximumfocus-versus alignment sensitivity.

A typical value for the optimal focus offset FO for a projectionapparatus having a NA=1.2 is about −120 nm which would have a usablerange of about −200 nm.

After the exposure of the verification fields, the test substrate isdeveloped and then an alignment offset for each of the verificationmarkers is measured using for example a standard alignment sensorpresent in the system. Next, the measured alignment offsets for each ofthe verification markers are translated into defocus data using what iscalled a transposed focal curve. In an embodiment this transposed focalcurve is determined by transposing a section of the focal curve of thelithographic apparatus corresponding to the working range 42, see FIG.4. The transposed focal curve is in fact the focus sensitivity of theapparatus.

In an embodiment, after the verification fields have been exposed,additional calibration fields are exposed. In an embodiment, thecalibration fields are exposed with a small shift with respect to theverification fields. The shift prevents the marks from overlapping. Inan embodiment, the calibration fields are fields exposed with predefinedRx, Ry tilt offsets per field. The tilts will make rows/columns ofcalibration marks within the calibration fields to be exposed atdifferent focus height. In FIG. 5, an example of the Z position of thecalibration marks Mk1 . . . Mk13 of one particular row are shown as afunction of the X-position. In FIG. 5 a Ry tilt (that is a rotation ofthe field around the Y axis) of a substrate surface 50 with respect toan image plane 51 is introduced by a tilt of the substrate W. In FIG. 5part of a lens 52 is shown which defines the position of the image plane51. The Ry tilt of the substrate W around the Y axis will introduce a Zdefocus for FOCAL markers in each of the rows of the calibration field,(i.e. Mk1 . . . Mk13, Mk14 . . . Mk26, . . . , Mk234 . . . Mk247) atdifferent X positions. FIG. 6 shows a graph of a measured alignmentoffset for one single row in field X positions of calibration marks ofone calibration field. The calibration fields 71 may be exposed atcenter positions of a substrate 70, see FIG. 7. Exposing with shifts (toavoid overlapping of exposed marks) at centre of position of samesubstrate is desirable get most reliable data.

By exposing a plurality of calibration fields with tilts in Rx, Ry andsmall offsets, accurate focus calibration curves can be derived with asmooth distribution of focus data points. An example of such acalibration curve 80 is shown in FIG. 8. The calibration curve 80 can beused to convert the full wafer verification fields information into adefocus map. To do this part of the calibration curve is selected andtransposed. This will result in a sensitivity curve dZ=S(dX,dY) whichcan be used for calculating the dZ for each mark in the test substrategiven the (measured) values for dX and dY.

It should be noted that instead of using a Rx, Ry tilt for the exposureof the calibration marks, the calibration marks may also be exposed witha focus offset per field.

By measuring defocus using the method according to an embodiment of theinvention, a focus versus alignment shift sensitivity of up to 50 timeshigher than a conventional LVT may be obtained, typically d(X,Y)/dZ=20.This sensitivity is well in line with future system needs as for signalto (alignment) noise ratio. Also, an embodiment of the presented methoduses a standard binary mask which images standard FOCAL marks. Hence,this method will be fully compatible with future ways of illumination(as in maskless and EUV). Standard binary mask are generally easy tomanufacture with relative low costs. An embodiment of the presentedmethod has a higher spatial distribution of marks than the state of theart methods; 247 verification marks per field compared to 55verification marks of LVT, which will result a higher resolution of thedefocus maps.

It is noted that apart from providing leveling focus information,embodiments of methods according to the invention can be used for:

Chuck Deformation Map (CDM) calibration (with a higher spatial density)

2D Grid plate calibration (NXT) (requiring a high spatial density)

chuck-to-chuck focus plane offset ATP test

Image Plane Deviation (IPD) ATP test

Rx calibration (requiring a higher spatial density then supported byLVT)

Embodiments may also be suitable for investigating fluid lenstemperature impact on focus. By exposing at different numerical aperturedifferent temperature sensitivity can be obtained.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g. having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, the invention may take the form of acomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g. semiconductor memory, magnetic or optical disk) having sucha computer program stored therein.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

The invention claimed is:
 1. A method of measuring the focus of alithographic projection apparatus, said method comprising: covering atest substrate with photo resist; placing said test substrate on asubstrate table of said lithographic apparatus; exposing a plurality ofverification fields on said test substrate, each of said verificationfields comprising a plurality of verification markers, wherein saidverification fields are exposed using a predetermined focus offset in apredetermined focal curve; exposing a plurality of calibration fields onsaid test substrate, each of said calibration fields comprising aplurality of calibration markers, said calibration fields being exposedwith predefined tilt offsets per calibration field; developing saidphoto resist; measuring an alignment offset for each of saidverification markers; measuring an alignment offset for each of saidplurality of calibration markers to render calibration data; determininga calibration curve using said calibration data; making a referencefocal curve by using a portion of said calibration curve; translatingthe measured alignment offsets for each of said verification markersinto defocus data using the reference focal curve.
 2. A method accordingto claim 1, wherein said predetermined focus offset is selected so thata local value of said predetermined focal curve of said lithographicapparatus is less than a maximum value of said predetermined focalcurve.
 3. A method according to claim 2, wherein said predeterminedfocus offset FO is selected so that said predetermined focal curve ofsaid lithographic apparatus is substantially linear in a working rangearound said focus offset.
 4. A method according to 3, wherein saidcalibration fields are exposed with a shift in a plane with respect tosaid verification fields.
 5. A method of calibrating a lithographicprojection apparatus, said method comprising a method of measuring focusaccording to claim 1, wherein said defocus data is used to adjustsettings of said lithographic projection apparatus.
 6. A method ofcalibrating a lithographic projection apparatus, comprising: covering atest substrate with photo resist; placing the test substrate on asubstrate table of the lithographic apparatus; exposing a plurality ofverification fields on the test substrate, each of the verificationfields comprising a plurality of verification markers, wherein theverification fields are exposed using a predetermined focus offset in apredetermined focal curve; exposing a plurality of calibration fields onthe test substrate, each of the calibration fields comprising aplurality of calibration markers, the calibration fields being exposedwith predefined tilt offsets per calibration field; developing the photoresist; measuring an alignment offset for each of the verificationmarkers; measuring an alignment offset for each of the plurality ofcalibration markers to render calibration data; determining acalibration curve using the calibration data; making a reference focalcurve by using a portion of the calibration curve; translating themeasured alignment offsets for each of the verification markers intodefocus data using the reference focal curve; and using the defocus datato adjust settings of the lithographic projection apparatus.
 7. A methodaccording to claim 6, wherein the predetermined focus offset FO isselected so that the local value of the predetermined focal curve of thelithographic apparatus is less than a maximum value of the predeterminedfocal curve.
 8. A method according to claim 7, wherein the predeterminedfocus offset FO is selected so that the predetermined focal curve of thelithographic apparatus is substantially linear in a working range aroundthe focus offset.
 9. A method according to claim 6, wherein thecalibration fields are exposed with a shift in the plane with respect tothe verification fields.